It seems very likely that CFU is a term that refers to a single cell, that cells with the capacity to form hemopoietic nodules in the spleen of irradiated mice (CFC) can be separated on the basis of other properties (e.g. density), and that pluripotent hemopoietic stem cells (SC) may be one of the various classes of CFC. We propose to study the question of whether CFC circulate continuously when transplanted into irradiated mice, whether CFC separated on the basis of size and density have different circulation kinetics as well as different proliferative potential, and whether CFC that are dividing are relatively "fixed" in tissue while CFC not in active phases of the cell cycle are free to circulate. We wish also to study the long range potential of these cells for hemopoietic repopulation and repair, as measured by 100 day survival in lethally irradiated mice given different classes of CFC. C3H mice will serve as donors of hemopoietic tissue and other mice of this strain will be exposed to total-body x-radiation followed by transfusion of cells. Separation techniques involving sedimentation in density gradients will be used to obtain CFC with different physical properties. Cells from bone marrow and from spleen will be compared with respect to: 1) separation in density gradients, 2) number and histologic types of spleen colonies at 9 days, 3) ability to protect lethally irradiated mice for 100 days, and 4) rate of recovery in peripheral WBC, platelets and RBC in the mice surviving lethal irradiation. The results should help clarify the concepts of CFU, CFC, and SC and permit a better understanding of the interrelationships among these concepts and the cellular entities to which the concepts refer.